DNA Replication How DNA makes an exact copy of itself.

During the cell cycle when a cell is ready to divide, it must copy or Copying occurs during In the After the DNA is copied, it supercoils into chromosomes so that exact copies of the DNA is in each new daughter cell

DNA Structure Review Two polynucleotide chains, held together by Pairing: Adenine Guanine

Because bases pair together, each strand of DNA can be used as a template to make another strand This occurs in two steps: 1. The enzyme “unzips” the DNA strand, breaking the hydrogen bonds between the bases Helicase

2. The enzyme DNA creates a new strand along each side of the molecule by matching up base pairs *Notice that the two strands are EXACT copies of each other

Semiconservative Replication When the two new strands are complete, each strand consists of One strand that is from the One newly constructed strand *This combination of old and new strands is called semiconservative replication

Meselson and Stahl’s Experiment: Proof of the semiconservative model Three theories about how DNA replicates before the semi- conservative model was proved:

15 N = Nitrogen Isotope (Heavier) 14 N = Nitrogen Isotope (Lighter) DNA made from the heavier 15 N will sink DNA made from the lighter 14 N will rest DNA made from both 14 N and 15 N will rest in the

Two bands in the final test tube, one midway and one at the top: the results of the experiment. Two bands in the final test tube, one at the bottom, one at the top Two bands in the test tube, both in different levels in the middle

Click for Animation of Experiment Or copy and paste the link: com/olcweb/cgi/pluginpop.cg i?it=swf::535::535::/sites/dl/fr ee/ /120076/bio2 2.swf::Meselson and Stahl Experiment